Amphibian chytridiomycosis

For more than 20 years, ZSL has been at the forefront of research on amphibian chytridiomycosis, a disease which has devastated amphibian populations globally.

Video of Battling one of the fastest species declines on record | Wild Science

Summary of the research

The global decline of amphibians is one of the most important biodiversity crises facing us today. Amphibian declines have many causes, but until the late 20th century infectious disease had not been identified as a factor. In the mid-1990s, however, an international team of scientists led by Professor Andrew Cunningham, a researcher from ZSL’s Institute of Zoology (IoZ), identified a novel infectious agent, Batrachochytrium dendrobatidis (Bd), a chytrid fungus that produces the amphibian disease chytridiomycosis, and demonstrated that this was a major cause of amphibian mortality and declines on at least two continents.

At this time, Prof. Cunningham was Chair of the Pathology & Disease Working Group of the IUCN’s Declining Amphibian Population Task Force, which enabled him to bring together amphibian decline researchers and pathologists across the world and which resulted in this discovery; an effort described as "an exemplary example of international scientific collaboration" (Nature 394, 418-419). Prof. Cunningham and the team were each awarded a medal by the CSIRO in Australia for this work.

Subsequently, IoZ’s scientists have spent close to two decades working on this disease, including collaborating with researchers from dozens of countries to understand the current status and underlying drivers of the distribution, spread and impacts of Bd. In particular, we have described infection and lethal impacts in dozens of amphibian species across Europe, the Caribbean, South America, Africa and Asia.

Mountain chicken frog

As the amphibian decline crisis deepened, a ZSL amphibian working group was established to bring staff together from across the organisation to maximise our efforts to combat amphibian declines through research, in-country conservation and capacity building and conservation breeding in ZSL London Zoo. Some of our zoo-bred animals, housed under biosecure conditions, have been released back into the wild with careful post-release monitoring to learn from successes and failures alike. In all these areas we work closely with other institutions, including universities, governments and other conservation non-governmental organisations.

Our work and other studies have shown that the mortality caused by chytridiomycosis can cause catastrophic and persistent amphibian population declines that may, in extreme cases, result in species extinctions. Collectively, this has identified environmental conditions and amphibian taxonomic groups that are at greater risk of disease emergence and decline due to chytridiomycosis. Using state-of-the-art genome sequencing technology, we have shown how the recent evolution of Bd produced a novel, hypervirulent and now globally-distributed form of the pathogen that is predominantly responsible for amphibian mortality and declines worldwide.

We continue to identify new variants of Bd using next-generation sequencing technologies and we are currently investigating how competition amongst genetic variants may be behind the evolution of hypervirulence. In addition, ZSL staff were involved in the recent discovery of a completely new species of chytrid fungus that is devastating fire salamander populations in Western Europe and we have been at the forefront of working with the British government to try to mitigate any impacts this new disease might have on amphibians in the UK.

We are constantly translating our world-class research on chytridiomycosis into conservation strategies that combat this threat to amphibians. IoZ scientists have shown how introduced and non-native species may commonly harbour infections and how movement of amphibians for trade and even conservation purposes can expand the range of Bd and other pathogens.

We work with national and international entities to develop strategies that our research indicates will reduce the risk of infection being spread by the amphibian trade and reduce the risk of spill-over from captive to wild populations. We are at the forefront of amphibian welfare, developing methods for treating infection in both captive and wild populations. Our latest work takes our welfare research into the wildlife conservation setting: we are the first to publish evidence that infection can be cleared from wild amphibian populations.

Other material

Key achievements

Research published in 1998, co-authored by IoZ’s Professor Andrew Cunningham, was the first description of chytridiomycosis in amphibians and showed that this disease is responsible for mass mortality of amphibians across Australia and the Neotropics. Since then, our scientists and their colleagues have shown how the impact of amphibian chytridiomycosis is context-dependent, reliant on the amphibian species infected, the strain of Bd present and the prevailing environmental conditions.

IoZ’s Prof. Cunningham was one of four international scientists who reviewed available knowledge of amphibian chytridiomycosis for the World Organisation for Animal Health (OIE). This review led to infection withBd being listed by the OIE which means that 178 OIE member countries are signed up to new surveillance and control measures, as well as trade standards to help assure the sanitary safety of international trade in live amphibians and their products.

IoZ scientists, in collaboration with Imperial College, established the first tool for mapping cases of Bd infection globally.

IoZ developed biosecurity measures for use in amphibian conservation programmes, which are now employed in the Caribbean, across Europe, and in China as best practice guidelines for field researchers working with amphibians in the wild. IoZ staff and students have explored several antifungal treatments for Bd for captive animals. The successful treatments are now used by zoos and others across the world.

A seven-year study led by scientists from the IoZ, the National Museum of Natural History in Spain (MNCN), and Imperial College London showed the first evidence of eradicating Bd affecting amphibians in situ. The study combined antifungal treatment of Mallorcan midwife toad (Alytes muletensis) tadpoles with environmental disinfection. By using an antifungal to treat tadpoles and a common laboratory decontaminant to sterilise the environment, researchers were able to clear infection from populations of the toad over the research period. Read more.

Amphibians have been treated in the wild for the first time against chytridiomycosis as part of a pioneering study led by scientists from ZSL. Read more.

Latest research

January 2016: A study co-authored by IoZ’s Professor Andrew Cunningham and PhD student Michael Hudson tested the in-situ treatment of individual mountain chicken frogs using the antifungal drug, itraconazole. They found increased probability of survival and loss of the fungus infection for treated frogs compared to non-treated. Results suggest that in-situ treatment of individuals with the drug could be a useful short term measure to reduce the chtridiomycosis-induced mortality rate and to increase other conservation actions, or facilitate population survival during periods of high disease risk.

August 2016: Findings from research co-authored by IoZ’s Professor Andrew Cunningham and PhD student Michael Hudson suggest that the decline of the mountain chicken across its range is amongst the fastest recorded for any species, with island-wide population collapses due to chytridiomycosis occurring within 18 months on Dominica and under one year on Montserrat. There’s an urgent need to build mitigation capacity where amphibians are at risk from chytridiomycosis and to prevent the spread of the disease to new areas.

What’s next?

We continue to:

Develop further safe treatments to cure the disease in captive animals and wild populations

Determine whether field treatments can be used to reduce the impact of the disease on wild amphibians in various settings and populations.

Identify new genetic variants of chytrid fungi to understand what genetic factors are responsible for hyper- and hypo-virulence

Determine whether restocking is a useful conservation tool following declines due to chytridiomycosis

Identify how human activities like wildlife trade can be modified to reduce the risk of transmitting lethal infections to captive animals and wild populations

Transfer our research knowledge and skills to individuals and organizations who join the effort to manage chytridiomycosis.